Despite progress in the detection and treatment of metastatic breast cancer, mortality from this disease remains high because current therapies are limited by the emergence of cancer cells that are resistant to treatment and capable of metastatic progression. Increasing evidence suggests that these cells develop, in part, because they behave like stem cells and thereby evade cell death induced by therapies which target rapidly dividing tumor cells. This proposal is directed at elucidating molecular pathways important in metastatic progression and "stemness" in breast cancer with the goal of identifying novel therapeutic targets and biomarkers. Our focus is the HMGA1 oncogene because recent findings suggest that it plays a critical role in both of these processes. This gene encodes the HMGA1a and HMGA1b chromatin binding proteins, which function in modulating gene expression. HMGA1 is highly expressed during embryogenesis, but not in adult tissues. Strikingly, HMGA1 is also overexpressed in virtually all high-grade (poorly differentiated) human cancers studied to date. We first established that HMGA1 induces oncogenic transformation in cultured cells derived from normal breast cells. HMGA1 also causes aggressive cancers in transgenic mice and promotes an epithelial-to-mesenchymal transition (EMT) in MCF-7 breast cells. Conversely, inhibiting its expression blocks transformation phenotypes in high-grade, human breast cancer cell lines and prevents metastatic progression in some tumor models. A recent study also found that HMGA1 is among a list of 9 core transcription factors enriched in high-grade/poorly differentiated breast cancers and normal embryonic stem cells, further implicating HMGA1 as a key regulator in breast cancer progression and stem cells. Taken together, these findings suggest that HMGA1 orchestrates transcriptional networks that maintain a primitive, poorly differentiated state, both in breast cancer and stem cells. Based on these findings, we hypothesize that HMGA1 drives tumor progression by inducing transcriptional networks that maintain an undifferentiated, "stem-like" phenotype. Here, we propose studies to determine if HMGA1 is a biomarker and potential therapeutic target in metastatic breast cancer. We will also begin studies to determine how HMGA1 drives tumor progression in breast cancer. Using our unique resources, we propose the following Specific Aims: 1.) Determine if HMGA1 can serve as a biomarker for more advanced, less differentiated breast cancer using a tissue microarray with primary breast tumors and detailed clinical data from >500 patients, 2.) Elucidate the role of HMGA1 in tumor progression and the stem-cell phenotype using gain-of- function/loss-of-function approaches, and, 3.) Identify the molecular signature of HMGA1 in metastatic breast cancer and begin to define the functional significance of downstream transcriptional targets. Results from our studies will elucidate novel molecular circuitry important in tumor progression and should lead to the discovery of cellular pathways that could be targeted in therapy for metastatic breast cancer. PUBLIC HEALTH RELEVANCE: Although metastatic breast cancer is a common and highly lethal cancer that affects women worldwide, the cellular pathways that mediate tumor progression and resistance to therapy are poorly understood. To address this knowledge gap, we propose to study the HMGA1 oncogene, which is highly expressed in advanced breast cancer and embryonic stem cells. Results from our studies should enhance our understanding of how breast cancer progresses and provide the basis to design better therapies directed at these resistant cells.